Final answer:
The correct option is D). Pyrimidine dimers in DNA are most commonly caused by exposure to ultraviolet light, which can lead to harmful mutations if not repaired through nucleotide excision.
Step-by-step explanation:
If you observe a pyrimidine dimer in DNA under a microscope, it is indicative that the DNA has likely been exposed to ultraviolet light (UV light). Pyrimidine dimers, specifically thymine dimers, result from exposure to UV radiation, which can cause adjacent pyrimidines on a DNA strand to become covalently linked. This linkage is harmful because, if left unrepaired, it can stall DNA replication and transcription, leading to mutations. Fortunately, such damage can often be repaired by a DNA repair process known as nucleotide excision.